Buddy,
I would tell her that your friend really has a crush on her and would die to sleep with her. He respects me and you too much to do anything about it. Even if we stopped seeing each other he still wouldn't. So I told him that maybe if you are interested we can mess around together some night. If you are not comfortable, you can always back out?
Let it drop for a while. She will think about it, fantasize about it and then setup a scenario to let it happen. Just starting making out with her while he is in the same room and see how she responds.
Yes, this sounds like a potentially good approach. Maybe.
I assume that this friend is your "best friend", whom you referred to in previous posts? The "closeted" dude, on whose computer you discovered gay porn, and you then accused him of being gay?
As I recall from your posts, your best friend is the one who initially suggested that you 2 have a 3-way with a girl?
I'm wondering if the 3-way with this girl wouldn't, in fact, be more about you 2 guys getting together in a sexual situation with each other? From your previous posts, it would appear, that you love your best buddy and have more sexual interest in him, than you do in this girl. I'm wondering if it's the same for him and this is more about sex with you, than with her, and this is just a way of keeping his(and your) straight cover intact?